The present application claims priority under 35 U.S.C. xcfx86119 to Japanese Application No.2000-196640 filed on Jun. 29, 2000, which is incorporated herein by reference in its entirely for all purposes.
1. Field of the Invention
The present invention relates to a frequency controlled oscillator.
2. Description of the Related Art
A frequency control oscillator is used generate a signal of a fixed frequency in communication equipment. It is desirable that the signal output from the frequency control oscillator has a constant frequency even if temperature varies. A conventional frequency control oscillator is disclosed in Japanese laid open number HEI 7-38433.
FIG. 1 is a schematic diagram of the frequency control oscillator disclosed above-mentioned reference. The frequency control oscillator has a voltage controlled oscillator 1, a duplicator 2, a low pass filter 3, a high pass filter 4, detectors 5 and 6, a differential voltage output circuit 7 and an output terminal 10.
The duplicator 2 produces three signals which are duplicates of an output signal from the voltage oscillator 1, which has a fixed frequency (for example 30 GHz), and outputs the three signals to the low pass filter 3, the high pass filter 4 and the output terminal 10, respectively. The low pass filter 3 selects a frequency component under 30 GHz, and outputs the same to the detector 5. The detector 5 detects an envelope of a low frequency component, and outputs a corresponding low frequency component signal to the differential voltage output circuit 7. The high pass filter 4 selects a frequency component over 30 GHz, and outputs the same to the detector 6. The detector 6 detects an envelope of a high frequency component, and outputs a corresponding high frequency component signal to the differential voltage output circuit 7. The differential voltage output circuit 7 calculates a difference between the low frequency component signal and the high frequency component signal, and outputs an error signal corresponding to the difference to the voltage control oscillator 1. The voltage control oscillator 1 controls an output frequency according to the error signal.
The operation of the conventional frequency control oscillator is described below. If the frequency of the output signal from the voltage control oscillator 1 is the target frequency (for example 30 GHz), then the low frequency component signal and the high frequency component signal are the same. The differential voltage output circuit 7 therefore outputs an error signal corresponding to xe2x80x9c0xe2x80x9dV, and the frequency of the output signal generated by the voltage control oscillator 1 is not changed. If the frequency of the output signal from the voltage control oscillator 1 is higher than the target frequency, a voltage of the high frequency component signal is higher than a voltage of the low frequency component signal. The differential voltage output circuit 7 therefore outputs an error signal corresponding to the difference, and the frequency of the output signal generated by the voltage control oscillator 1 is decreased according to the error signal. If the frequency of the output signal from the voltage control oscillator 1 is lower than the target frequency, the voltage of the low frequency component signal is higher than the voltage of the high frequency component signal. The differential voltage output circuit 7 therefore outputs an error signal corresponding to the difference, and the frequency of the output signal generated by the voltage control oscillator 1 is increased according to the error signal.
As described above, the high pass filter 4 and the low pass filter 3 are used to detect the difference between the output frequency and the target frequency, and the output frequency is controlled according to the detected difference.
However, the high pass filter 4 and low pass filter 3 must have precise operating characteristics in order to detect a slight difference between the frequency of the output signal and the target frequency. Therefore, the filters have a large area and multiple components, which makes downsizing difficult.
A frequency control oscillator includes an oscillator which generates an oscillation signal, and a frequency of the oscillation signal control led according to an error signal, a phase shifter which outputs a phase shift oscillation signal corresponding to a difference between the frequency of the oscillation signal and a target frequency, a multiplier which outputs a multiplied signal corresponding to a multiplied value of the phase shift signal and the oscillation signal and an error signal generator which outputs the error signal according to the multiplied signal.